1. Field of the Invention
The present invention relates to a light-emitting display, and more specifically, to an organic light-emitting display using luminescence of an organic material.
2. Discussion of the Background
Generally, an organic light-emitting display emits light with an organic light-emitting element that uses luminescence of an organic material. N×M organic light-emitting cells, arranged in a matrix form, may be driven with a voltage or current to display images. The organic light-emitting cell may also be called an organic LED (light-emitting diode) because it has diode characteristics, and it may include an anode (ITO), an organic thin film, and a cathode (metal). The organic thin film may have a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for balancing electrons and holes to improve luminescence efficiency. The organic thin film may further include an electron injecting layer (EIL) and a hole injecting layer (HIL).
Organic light-emitting cells may be driven by a passive matrix driving method or an active matrix driving method, which may use a thin film transistor (TFT) or a MOSFET. The passive matrix organic EL display may be constructed having an anode and a cathode that are perpendicular to each other, and a line may be selected to drive the light-emitting cells. The active matrix display may comprise a TFT coupled to each ITO pixel electrode, and it may be driven by a voltage maintained by a capacitor coupled to the gate of the TFT.
A conventional active matrix organic light-emitting display will now be explained.
FIG. 1 is an equivalent circuit diagram showing a pixel of a conventional active matrix organic light-emitting display. Referring to FIG. 1, the pixel circuit may include an organic LED OLED, a switching transistor SM, a driving transistor DM, and a capacitor Cst. The two transistors SM and DM may be PMOS transistors.
When the switching transistor SM turns on in response to a select signal applied to its gate from a signal line Sn, a data voltage VDATA from a data line Dm is supplied to the gate of the driving transistor DM. Then, a current IOLED, corresponding to a voltage VGS charged between the gate and source of the driving transistor DM according to the capacitor Cst, may flow through the driving transistor DM, thereby causing the organic LED OLED to emit light. Here, the current IOLED may be represented by Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In the pixel circuit of FIG. 1, a current corresponding to the data voltage may be supplied to the organic LED, thereby causing it to emit light with at a luminance corresponding to the current. The data voltage may have multiple values in a specific range in order to represent a predetermined gray scale.
As Equation 1 shows, however, the current IOLED varies with the threshold voltage VTH of the driving transistor DM. Accordingly, the organic light-emitting display may not display correct images because the driving transistors of the pixels may have different threshold voltages.